1. Field of the Invention
The present invention is directed to wireless communication systems and, more particularly, to multiplexing transmissions conveying large payloads of control information from user equipments.
2. Description of the Related Art
A communication system consists of a DownLink (DL), supporting transmissions of signals from a base station (Node B) to User Equipments (UEs), and of an UpLink (UL), supporting transmissions of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
The DL signals consist of data signals, carrying the information content, control signals providing Downlink Control Information (DCI), and Reference Signals (RS) which are also known as pilots. The Node B transmits DCI through a Physical Downlink Control CHannel (PDCCH) and data information through a Physical Downlink Shared CHannel (PDSCH).
The UL signals consist of data signals, carrying the information content, control signals providing Uplink Control Information (UCI), and Reference Signals (RS). The UEs convey UL data signals through a Physical Uplink Shared CHannel (PUSCH). UCI signals include acknowledgement signals associated with the application of a Hybrid Automatic Repeat reQuest (HARQ) process, Service Request (SR) signals, Channel Quality Indicator (CQI) signals, Precoding Matrix Indicator (PMI) signals, and Rank Indicator (RI) signals. The combination of CQI, PMI, and RI will be referred to as Channel State Information (CSI). UCI can be transmitted in a Physical Uplink Control CHannel (PUCCH) or, together with data, in the PUSCH.
The CSI is used to inform the Node B of the channel conditions the UE experiences in the DL in order for the Node B to select the appropriate parameters, such as the Modulation and Coding Scheme (MCS), for the PDCCH or PDSCH transmission to the UE and ensure a desired BLock Error Rate (BLER) for the respective information. The CQI provides a measure of the Signal to Interference and Noise Ratio (SINR) over sub-bands or over the whole operating DL BandWidth (BW), typically in the form of the highest MCS for which a predetermined BLER target can be achieved for a signal transmission by the Node B in the respective BW. The PMI and RI are used to inform the Node B how to combine a signal transmission to the UE from multiple Node B antennas in accordance with the Multiple-Input Multiple-Output (MIMO) principle. Full channel state information in the form of channel coefficients allows the selection of the precoding weights with MIMO to closely match the channel experienced by the UE and offer improved DL performance at the expense of increased UL overhead required to feedback the channel coefficients relative to other CSI signal types.
An exemplary structure for the PUSCH transmission in the UL Transmission Time Interval (TTI), which for simplicity is assumed to consist of one sub-frame, is shown in FIG. 1. The sub-frame 110 includes two slots. Each slot 120 includes symbols used for the transmission of data signals, control signals, or RS. Each symbol 130 further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The PUSCH transmission in one slot may be located at the same or at a different part of the operating BW than the PUSCH transmission in the other slot. Some symbols in each slot can be used for RS transmission 140 to provide channel estimation and enable coherent demodulation of the received signal. The transmission BW is assumed to consist of frequency resource units which will be referred to as Physical Resource Blocks (PRBs). Each PRB is further assumed to consist of NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPUSCH PRBs 150 for PUSCH transmission for a total of MscPUSCH=MPUSCH·NscRB REs for the PUSCH transmission BW.
An exemplary UE transmitter block diagram for UCI and data transmission in the same PUSCH sub-frame is illustrated in FIG. 2. Coded CQI bits 205 and coded data bits 210 are multiplexed 220. If HARQ-ACK bits also need to be multiplexed, data bits are punctured to accommodate HARQ-ACK bits. Discrete Fourier Transform (DFT) of the combined data bits is performed in DFT unit 230. UCI bits are then obtained by performing sub-carrier mapping in sub-carrier mapping unit 240, wherein the REs corresponding to the assigned transmission BW are selected in control unit 250. Inverse Fast Fourier Transform (IFFT) is performed in the IFFT unit 260. Finally the CP is inserted in CP Insertion unit 270 and filtering is performed in Time Windowing unit 280, which outputs the transmitted signal 290. For brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated. Also, the encoding process for the data bits and the CSI bits, as well as the modulation process for all transmitted bits, are omitted for brevity. The PUSCH signal transmission is assumed to be over clusters of contiguous REs in accordance to the DFT Spread Orthogonal Frequency Multiple Access (DFT-S-OFDM) method allowing signal transmission over one cluster 295A (also known as Single-Carrier Frequency Division Multiple Access (SC-FDMA)), or over multiple non-contiguous clusters of contiguous BW 295B.
The Node B receiver performs the reverse (complementary) operations of the UE transmitter. This is conceptually illustrated in FIG. 3 where the reverse operations of those illustrated in FIG. 2 are performed. After an antenna receives the Radio-Frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) which are not shown for brevity, the received signal 310 is filtered in Time Windowing unit 320 and the CP is removed in CP Removal unit 330. Subsequently, the Node B receiver applies a Fast Fourier Transform (FFT) in FFT unit 340, selects the REs used by the UE transmitter in Sub-Carrier Demapping unit 350, applies an Inverse DFT (IDFT) in IDFT unit 360, extracts the HARQ-ACK bits and places respective erasures for the data bits in Extraction unit 370, and de-multiplexes in Demultiplexer unit 380 the data bits 390 and CSI bits 395. As for the UE transmitter, well known Node B receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity.
An exemplary structure for the CSI transmission in one slot of the PUCCH is illustrated in FIG. 4. A similar structure may also be used for the HARQ-ACK transmission in the PUCCH. The transmission in the other slot, which may be at a different part of the operating BW for frequency diversity, is assumed to effectively have the same structure. The CSI signal transmission in the PUCCH is assumed to be in one PRB. The CSI transmission structure 410 comprises the transmission of CSI signals and RS for enabling coherent demodulation of the CSI signals. The CQI bits 420 are modulated in modulators 430 with a “Constant Amplitude Zero Auto-Correlation (CAZAC)” sequence 440, for example with QPSK modulation, which is then transmitted after performing the IFFT operation as it is subsequently described. Each RS 450 is transmitted through the unmodulated CAZAC sequence.
An example of CAZAC sequences is given by Equation (1).
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                  n                        +                        1                                            2                                                                      )                                      ]                                              (        1        )            
In Equation (1), L is the length of the CAZAC sequence, n is the index of an element of the sequence n={0, 1, . . . , L−1}, and k is the index of the sequence. If L is a prime integer, there are L−1 distinct sequences which are defined as k ranges in {0, 1, . . . , L−1}. If the PRBs comprise of an even number of REs, such as for example NscRB=12, CAZAC sequences with even length can be directly generated through computer search for sequences satisfying the CAZAC properties.
FIG. 5 shows an exemplary transmitter structure for a CAZAC sequence that can be used without modulation as RS or with modulation as CSI signal. The frequency-domain version of a computer generated CAZAC sequence, generated in CAZAC Sequence generator 510, is used. The REs corresponding to the assigned PUCCH BW are selected in Control unit 520 for mapping in Sub-Carrier Mapping unit 530 the CAZAC sequence. An IFFT is performed in IFFT unit 540, and a Cyclic Shift (CS), as it is subsequently described, is applied to the output in Cyclic Shift unit 550. Finally, the CP is inserted in CP Insertion unit 560 and filtering is performed in Time Windowing unit 570, which outputs transmitted signal 580. A UE is assumed to apply zero padding in REs used for signal transmission by other UEs and in guard REs (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not shown.
The reverse (complementary) transmitter functions are performed for the reception of the CAZAC sequence. This is conceptually illustrated in FIG. 6 where the reverse operations of those in FIG. 5 apply. An antenna receives RF analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal 610 is filtered in Time Windowing unit 620 and the CP is removed in CP Removal unit 630. Subsequently, the CS is restored in the CS Restore unit 640, and a Fast Fourier Transform (FFT) is performed in FFT unit 650. The transmitted REs are selected in Sub-Carrier Demapping unit 660 under control of Control unit 665. FIG. 6 also shows the subsequent correlation in Multiplier 670 with the replica of the CAZAC sequence produced in CAZAC Based Sequence unit 680. Finally, the output 690 is obtained which can then be passed to a channel estimation unit, such as a time-frequency interpolator, in case of a RS, or can to detect the transmitted information, in case the CAZAC sequence is modulated by CSI information bits.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different CSs of the same CAZAC sequence can be allocated to different UEs in the same PRB for their RS or CSI transmission and achieve orthogonal UE multiplexing. This principle is illustrated in FIG. 7. In order for the multiple CAZAC sequences 710, 730, 750, 770 generated respectively from multiple CSs 720, 740, 760, 780 of the same root CAZAC sequence to be orthogonal, the CS value 790 should exceed the channel propagation delay spread DELTA (including a time uncertainty error and filter spillover effects). If TS is the DFT-S-OFDM symbol duration, the number of such CSs is equal to the mathematical floor of the ratio TS/D.
For the CSI transmission structure in the PUCCH sub-frame, as illustrated in FIG. 4, for one of the two sub-frame slots, 5 symbols carry CSI and 2 symbols carry RS. As the CSI transmission needs to be relatively reliable and cannot utilize HARQ retransmissions, it needs to be protected through reliable channel coding. As a result, the CSI payload supported in the PUCCH is small. For example, puncturing a (32, 10) Reed-Mueller (RM) code to (20, 10), 10 CSI bits can be transmitted using QPSK modulation and coding rate of ½ (20 coded bits).
In order to support higher data rates than possible in legacy communication systems, aggregation of multiple Component Carriers (CCs) can be used in both the DL and the UL to provide higher operating BWs. For example, to support communication over 100 MHz, aggregation of five 20 MHz CCs can be used. UEs capable of operating only over a single DL/UL CC pair will be referred to as “Legacy-UEs (L-UEs)” while UEs capable of operating over multiple DL/UL CCs will be referred to as “Advanced-UEs (A-UEs)”. The invention assumes that if an A-UE receives PDSCH in multiple DL CCs or transmits PUSCH in multiple UL CCs, a different data packet having its own HARQ process is conveyed by each such PDSCH or PUSCH transmission.
FIG. 8 further illustrates the principle of CC aggregation. An operating DL BW of 100 MHz 810 is constructed by the aggregation of 5 (contiguous, for simplicity) DL CCs, 821, 822, 823, 824, 825, each having a BW of 20 MHz. Similarly, an operating UL BW of 100 MHz 820 is constructed by the aggregation of 5 UL CCs, 831, 832, 833, 834, 835, each having a BW of 20 MHz. Each DL CC is assumed to be uniquely mapped to a UL CC (symmetric CC aggregation) but it is also possible for more than 1 DL CC to be mapped to a single UL CC or for more than 1 UL CC to be mapped to a single DL CC (asymmetric CC aggregation, not shown for brevity). The link between DL CCs and UL CCs can be UE-specific.
To improve cell coverage and increase cell-edge data rates in the DL, Coordinated Multiple Point (CoMP) transmission/reception through Joint Processing (JP) can be used where multiple Node Bs transmit the same data signal to a UE. The DL CoMP principle is illustrated in FIG. 9 wherein two Node Bs, Node B1 910 and Node B2 920, transmit a first data signal to UE1 930 and a second data signal to UE2 940. The Node Bs share the information content for a UE operating in DL CoMP mode through backhaul which is typically referred to as X2 interface 950. The backhaul, for example, may be a fiber optic link or a microwave link.
To support PDSCH reception by an A-UE over multiple DL CCs, the A-UE should be able to transmit substantially larger CSI or HARQ-ACK information payloads to the Node B than an L-UE having PDSCH reception only in a single DL CC. While for symmetric DL/UL CC aggregations UCI transmission may fundamentally appear just as a parallelization of the one for single DL/UL CC pair to multiple DL/UL CC pairs, it is instead preferable for a UE to transmit all UCI in only one UL CC. This allows addressing all possible UE-specific symmetric or asymmetric DL/UL CC aggregations with a single design. It also avoids Transmission Power Control (TPC) problems that may occur if the UE simultaneously transmits UCI signals with substantially different powers in different UL CCs. Therefore, it is advantageous to consider UCI signaling structures where the transmission is in a single UL CC. This further necessitates the transmission of larger UCI payloads, than legacy ones, in a single channel.
DL CoMP also represents a challenging scenario with respect to required payloads of CSI feedback signaling. Even for the benign case of CQI-only feedback, the respective payload increases proportionally with the number of Node Bs in the CoMP CSI reporting set. For example, for 3 Node Bs in the CoMP CSI reporting set, the total CSI payload is 30 bits and cannot be supported by the PUCCH structure in FIG. 4. Since a CoMP UE is often likely to also experience low UL SINR, supporting higher CSI payloads becomes even more challenging. The combination of CoMP and multiple DL CCs further increases the required CSI payloads.
The PUSCH can support substantially larger payloads than the PUCCH and can accommodate the increased UCI payloads. However, as the minimum PUSCH granularity is 1 PRB, the UL overhead from using conventional PUSCH to support only UCI transmissions can become substantial even when this is done for a few UEs per sub-frame.
Therefore, there is a need to design PUCCH structures supporting large payloads for UCI signaling in a communication system supporting DL CC aggregation or DL CoMP.
There is another need to minimize the UL overhead corresponding to the transmission of large UCI payloads.
Finally, there is a need to efficiently manage the PUCCH and PUSCH resources while supporting the transmission of large UCI payloads.